1. Field of the Invention
This invention relates to the field of semiconductor devices, and more specifically, to a process and system designed to improve linewidth control in photolithographic patterning.
2. Background Information
In the field of semiconductor devices the desire to achieve faster devices entails the fabrication of smaller and smaller devices. As device characteristics get smaller it becomes increasingly more important to control linewidth variation in lithographic patterning.
Linewidth variation, depending upon the severity, can cause a device to have poor performance or cause the device to fail altogether. For example, linewidth variation in the patterning of gate layers can cause the gate to be formed too large or too small. Larger gates slow down the semiconductor device such that the device has poor performance. Smaller gates are faster, however, if a gate is too small (i.e. smaller than required by the specific design characteristics) it may result in punchthrough, which ultimately causes device failure.
Linewidth variation can be caused by many sources. One source, for example, is the optical proximity effect. The optical proximity effect causes systematic (i.e., not random) linewidth differences between "isolated" features and "nested" features. The term "isolated features" is used to describe lines that are not in the presence of (or are not surrounded by) other lines with similar features and are thus "isolated". The term "nested features" is used to describe lines that are in the presence of (or are surrounded by) other lines with similar features and are thus "nested" or "grouped". When isolated and nested features are patterned on the wafer using commercially available stepping or scanning microlithographic equipment, linewidth variance occurs even though the size of the isolated and nested features on the reticle are the same. There are several methods to reduce the optical proximity effect and improve linewidth control.
One method to improve linewidth control is to select a lithographic process that does not exhibit significant optical proximity effects, such as negative tone patterning. In negative tone patterning the portions of the resist exposed to light become insoluble. These insoluble portions remain behind and act as an etch mask to protect the underlying layer which will form the lines or gates being patterned. Although not well understood, it is a known empirical fact to those skilled in the art, that negative tone patterning exhibits a much smaller degree of optical proximity effects than positive tone patterning. The pattern media, i.e., "dark" field reticles and negative resist, used in negative tone patterning exhibit a much smaller degree of optical proximity effects than the more widely used pattern media, "clear" field reticle and positive resists, of positive tone patterning.
Although negative tone patterning is advantageous due to its lack of optical proximity effects, negative tone patterning also has many disadvantages that have made positive tone patterning more desirable for the last several generations of microphotolithography, for example, generations with device features below 1.50 microns. One such disadvantage is that negative resists tend to exhibit a mechanism known as swelling. That is, negative resists increase in volume, or "swell", as a result of penetration of the resist material by the developer solution. Such swelling causes the feature size of the pattern created in the resist to be altered. As an example, for design features smaller than 3 .mu.m, the change in feature size is unacceptably large compared to the specified dimensions. Positive resists do not exhibit swelling, due to a different dissolution mechanism during development of the resist, and are therefore desirable for design features smaller than 3 .mu.m.
Scumming effects are another disadvantage of negative resists. Scumming effects are caused when radiation, scattered off the projection optics, crosslinks a thin layer in the top surface of a negative resist and the thin layer becomes punctured and slides down between the features. As little as 1% scattered light has been observed to produce this unwanted mechanism in negative tone patterning. In positive tone patterning, such scattered light only results in a slight reduction of resist thickness, and no scumming effect is produced. As a result, of swelling and scumming, there are no commercially available high performance negative tone resists for microlithographic patterning of the gate layers for the more recent generations of semiconductor devices. More recent generations of gate layers have dimensions, for example, below 0.8 .mu.m, and require exposure tools with an exposure wavelength of, for example, approximately 365 nm.
Another way to reduce linewidth differences caused by optical proximity effects is to increase the value of the exposure tool partial coherence (.sigma.) employed in positive tone patterning. Partial coherence (.sigma.) of the lithographic equipment is defined as the ratio of the illuminator numerical aperture to the numerical aperture of the projection optics. There is a complex relationship between the partial coherence of the exposure tool and the ability of the exposure tool to pattern and control the linewidth of minimal features of the different kinds of device layers over varying process conditions.
The partial coherence value of commercial microlithographic patterning equipment is customarily defined by the tool manufacturer. As requirements for tighter linewidth tolerance increase, the value of the partial coherence of the microlithographic exposure tool has steadily shifted toward the higher values. For example, exposure tools with partial coherence values above 0.65 have become available on the market. Such a shift to exposure tools with higher partial coherence values is the likely result of manufacturers willingness to sacrifice "ultimate" patterning capabilities and some degree of the process latitude for the reduced proximity effects brought by systems with higher partial coherence values.
Unfortunately, an increase in a system's partial coherence value may result in the creation of new sources of linewidth variations. Although the higher partial coherence value of the exposure tool improves linewidth control via reduction of the proximity effect, higher partial coherence values give rise to other sources of linewidth variation, such as across-the-field linewidth variations.
As an example, 400 nm lines patterned in the i9C Nikon Stepper (available from Nikon Precision Incorporated, Belmont, Calif.) with partial coherence value equal to 0.68 exhibit systemic across the field linewidth variance on the order of 60 nm. Only 20 nm of this across-the-field linewidth variance can be accounted for through known and attributable causes such as, mask errors, illumination nonuniformity, resist processing nonuniformity, as well as existing system residual aberrations and flare. This means that the largest portion (40 nm) of across-the-field linewidth variation is induced by the changing partial coherence across the stepper field. Since it is customary in microlithography to incorporate a feature linewidth control (tolerance) range of approximately +/-10% of the feature linewidth, the tolerance range for a 400 nm wide gate is approximately +/-40 nm. Therefore, the presence of the 40 nm source of linewidth variance that was induced by the exposure tool alone, in this example, is clearly excessive with regard to a manufacturer's ability to stay within customary linewidth tolerance ranges.
Thus, what is needed is a method and system, useful with either positive or negative tone patterning, that minimizes the range of linewidth variations in microlithographic patterning, and a method and system for mapping the coherence conditions across a stepper field.